The present invention relates to a thermosensitive recording material, and more particularly to an improved thermosensitive recording material comprising a support, an intermediate layer including minute voids formed on the support, and a thermosensitive coloring layer formed on the intermediate layer.
It is conventionally known that a colorless or light-colored coloring leuco dye reacts with a color developer such as an organic acidic material to be colored upon application of heat thereto until the leuco dye and the color developer are fused. Thermosensitive recording materials utilizing this coloring reaction are conventionally known as are disclosed, for instance, in Japanese Patent Publication Nos. 43-4160 and 45-14039.
Recently, those thermosensitive recording sheets have been employed in a variety of fields, for instance, for use with recorders for measurement instruments, terminal printers for computers, facsimile apparatus, automatic ticket vending apparatus, and bar code readers.
In accordance with recent remarkable improvements in the application of the above-mentioned recording apparatus to a variety of new fields and the performance thereof, there is a great demand for thermosensitive recording sheets which can complement those improved apparatus. More specifically, there are demanded thermosensitive recording sheets capable of yielding sharp images with high density at a small amount of thermal energy with the advance of the thermal head speed. In addition, thermosensitive recording sheets are required to satisfy the sufficient thermal head matching properties such as not forming any dust from the recording material in the course of thermosensitive recording by use of the thermal head which comes into contact with the surface of the thermosensitive recording material and not sticking to the thermal head.
Color development on the thermosensitive recording sheet can be attained by the melting of a coloring leuco dye or a color developer or both of them and the reaction of the two to induce a color in the leuco dye under the application of thermal energy from the thermal head.
As a method of increasing the thermal coloring sensitivity of the thermosensitive recording material, there is widely known a method of adding to the thermosensitive recording material a thermofusible material which melts at a temperature lower than the melting points of the employed leuco dye and the color developer and is capable of dissolving therein the leuco dye and color developer when melted.
A variety of thermofusible materials for use in this method have been proposed, for example, nitrogen-containing compounds such as acetamide, stearamide, m-nitroaniline, and phthalic acid dinitrile in Japanese Laid-Open Patent Application No. 49-34842; acetoacetic anilide in Japanese Laid-Open Patent Application No. 52-106746; and alkylated biphenyl alkanes in Japanese Laid-Open Patent Application No. 53-39139.
However, in accordance with the recent development of a high speed thermal facsimile apparatus, it has become a common practice to energize and drive a thermal head at high speed. Under such circumstances, there is a problem of undesirable color development that the background of a thermosensitive recording sheet is also colored due to the build-up of thermal energy in the thermal head and therearound in the course of repetition of high speed thermosensitive recording. Therefore it is one of the most important subjects to be cleared in order to enhance the dynamic thermal coloring sensitivity in the conventional thermosensitive recording materials, without decreasing the coloring initiation temperature thereof. In the conventional thermosensitive recording materials, the dynamic thermal coloring sensitivity cannot be increased unless a large amount of the above-mentioned thermofusible compounds is added, although the static thermal coloring sensitivity can be increased to some extent by the addition of the thermofusible compounds. However, when a large amount of the thermofusible materials is employed, the sticking and dust-adhesion problems occur in the course of thermal recording by a thermal head. Further, when the melting points of the thermofusible compounds are extremely low, the preservability of the thermosensitive recording sheets is significantly degraded so that the fogging of the background of the recording material occurs during the storage thereof.
For the purpose of advancing the dynamic thermal coloring sensitivity of the recording material, there have been proposed a method of improving the smoothness of the thermosensitive coloring layer, and a method of increasing the concentration of the coloring components by decreasing the relative amounts of components which do not contribute to the coloring reaction of the thermosensitive recording material, such as fillers and binder agents.
The method of improving the smoothness of the thermosensitive coloring layer is usually carried out without difficulty by use of a super calender. However, this method has the shortcomings that the background of the thermosensitive coloring layer is colored during the calendering process and the surface of the thermosensitive coloring layer becomes so glossy that the appearance of the recording material is impaired.
To the thermosensitive coloring layer, a filler such as calcium carbonate, clay, and urea-formaldehyde resin may be added to maintain the whiteness of the background of the coloring layer and to prevent the sticking and dust adhesion problems during the thermosensitive recording using a thermal head. Further a water-soluble binder agent may be added to the thermosensitive coloring layer to firmly bind the coloring components and other additives of the thermosensitive coloring layer to a support. When the amount of such a filler and a water-soluble binder agent is reduced, the desired properties for the thermosensitive recording material cannot be obtained.
Under such circumstances, a thermosensitive recording material has been proposed, in which an intermediate layer including numerous minute voids is interposed between a support and a thermosensitive coloring layer for the purpose of effectively utilizing the thermal energy provided by a thermal head. For example, Japanese Laid-Open Patent Application No. 59-5093 discloses a thermosensitive recording material including a layer comprising as the main component microballoon-like particles, which is interposed between a support and a thermosensitive coloring layer. Japanese Laid-Open Patent Application No. 59-171685 discloses a thermosensitive recording material wherein a layer containing a blowing agent and a thermoplastic polymeric material is formed on a support, which layer becomes a heat insulating layer when it is heated and a blowing gas is produced from the blowing agent. Further Japanese Laid-Open Patent Application No. 59-225987 discloses a thermosensitive recording material comprising a support, an expanded layer which is formed by coating an expandable plastic on the support and expanding the plastic with application of heat thereto, an undercoat layer comprising a filler and a binder agent, and a thermosensitive coloring layer, which layers are successively overlaid on the support in this order.
However, these thermosensitive recording materials have the drawbacks that heat insulating layer does not have satisfactory flexibility and heat insulating properties, and the close contact between a thermal head and a thermosensitive recording sheet is not sufficient.